Semiconductor manufacturing processes involve the use of chemical reagents that must meet strict purity requirements. These liquid chemical reagents are typically contained in sealed containers (e.g., ampoules) to protect against contamination of the chemical reagents and to prevent leakage. The chemical reagents typically require metal containers and container fittings that use metal-on-metal seals to avoid corrosion, contamination, or leakage under elevated pressures. When using a chemical reagent stored in such a container, it is often necessary to be able to determine the amount of chemical reagent left in the container without exposing the chemical reagent to the environment or exposing an operator to the chemical reagent.
Ultrasonic probes are commonly used in the semiconductor industry to measure the level of chemical reagent within a sealed container. A typical design includes multiple ultrasonic sensors positioned in a series along the length of a conduit within the probe, such as the sensors and configuration disclosed in U.S. Pat. No. 5,663,503 to Dam et al. A signal processing device (e.g., a controller, meter, personal computer, etc.) transmits electronic signals to the ultrasonic sensors, which in turn generate bursts of sound waves that pass through the conduit and echo back to the sensors. Each sensor converts the echoed waves it receives into electronic signals that are transmitted back to the signal processing device. The signal processing device then interprets the electronic signals to determine the intensity of the echoed waves as well as the time that elapsed between emission and the arrival of the echoed waves. For each sensor positioned along a particular portion of the conduit, the speed with which the ultrasonic waves travel through the conduit and the intensity of the echoed ultrasonic wave will differ depending on whether that portion of the conduit contains chemical reagent or gas or vapor (i.e., sound travels faster through a liquid medium as compared to gas or vapor). In this manner, the signal processing device can determine the level of the chemical reagent along the length of the conduit and therefore the amount of chemical reagent within the container.
Generally, a greater number of ultrasonic sensors disposed within the ultrasonic probe translates into increased accuracy in measuring chemical reagent levels. However such sensors are limited in that they cannot accurately measure chemical reagent levels that are near or below the lower end of the probe. This presents a problem because of the exacting nature of semiconductor manufacturing processes and environments, the cost associated with the chemical reagents and the cost associated with disposing of leftover chemical reagent when cleaning the containers.
Accordingly, there is a need in the art for an ultrasonic probe having the ability to measure the level of small amounts of chemical reagent within the container.